1. Field of the Invention
This invention relates to a laser beam scanning system, and more particularly to a laser beam scanning system for scanning a medium by using a plurality of laser beams, each having a different wavelength, the laser beams being intensity-modulated by a corresponding color video signal and deflected by a deflection means to scan the medium to produce a color image thereon.
2. Description of the Prior Art
Image output systems in which a spot of laser light that is intensity-modulated by video signals is used to scan a medium such as a screen or film to display or record an image thereon feature a number of advantages, such as the brightness and convergence of the laser beam and its coherence, and they are in wide use in industrial and medical fields.
Using the laser beam to scan two-dimensionally, horizontally and vertically, and having the scanning rate correspond to the raster scan of an ordinary TV system enables a real-time video image to be obtained which is free of residual image and as such has a broader range of use and markedly improved operability.
In such a system, methods for deflecting the laser beam to scan horizontally or vertically include mechanically driven methods that employ a swinging mirror or a polygonal mirror or other rotating multifaceted mirror, and non-mechanical methods using acousto-optical deflectors and the like.
However, in the case of ordinary raster scanning, with the NTSC system the horizontal scanning frequency is 15.75 KHz and the vertical scanning frequency is 60 Hz, while in the case of high-definition television the horizontal scanning frequency is even higher. As a result, using a mechanical drive for vertical deflection of the laser beam provides sufficient tracking and control stability, but there have been no low-cost, reliable mechanical control methods for providing the horizontal deflection because of the very high frequency that is involved.
Mechanical drive systems that employ swinging mirrors or rotating multifaceted mirrors do not cause color dispersion even when using a plurality of laser beams each having a different wavelength, and while high scanning frequencies of 10 KHz or more are possible there are problems relating to the durability of the working life of the mechanisms, while wobble of the mirror axis and the like make it difficult to produce a precise raster. On the other hand, because methods using acousto-optical deflectors have no mechanically controlled parts, they are very reliable, but involve the following problems.
FIG. 6 shows the operation of an acousto-optical deflector 60; the acousto-optical deflector 60 is driven by a signal source 61. If the ultrasonic driving frequency is f, the wavelength of the incident laser beam is .lambda. and the ultrasonic velocity is v, the angle.theta. of the first-order diffraction obtained by the operation of the acousto-optical deflector is: EQU .theta..congruent..lambda.f/v
However, with such a deflection system, unlike a mechanically driven mirror type method, because the angle of diffraction is dependent on the wavelength of the incident beam, color dispersion occurs. For example, when the three-color R (red), G (green), B (blue) laser beams shown in FIG. 7 impinge on the acousto-optical deflector 60, the longer the wavelength, the larger the angle of diffraction becomes, so the color dispersion of the first-order diffraction is produced.
Therefore, when a laser source producing a plurality of laser beams is used to produce chromatic information a medium, there has been a drawback that the coloring of the images has been difficult.
In view of this problem, there are known techniques comprising the use of a plurality of acousto-optical deflectors for different laser beam wavelengths, and combining the beams following the deflection; or a single acousto-optical deflector is used to deflect a plurality of laser beams of different wavelengths, and the resultant color dispersion that is produced is corrected by means of optical elements such as mirrors and prisms. With either method the optical system becomes complex and costly, and it is also extremely difficult to construct a correcting optical system which is capable of perfectly combining a plurality of laser beams of different wavelengths over a wide range of deflection angles. In addition, the aforesaid type of correcting optical system is set for the wavelengths of the laser beams, making it difficult to readily adjust it should it become necessary to change to a laser source of a different wavelength.
In addition to acousto-optical deflectors, there have been developed non-mechanical laser-beam deflection devices which can provide high scanning frequencies, but most of these devices employ optical diffraction or refraction which in itself gives rise to color dispersion, so there are problems of resolution and reliability and few of the devices have been put to practical use.
Thus, a reliable laser beam scanning system that provides laser beam scanning compatible with ordinary TV raster scanning and can perform input processing of color images in real time has not yet appeared.